1.
Optics
–
Optics is the branch of physics which involves the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviolet, and infrared light, because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties. Most optical phenomena can be accounted for using the classical description of light. Complete electromagnetic descriptions of light are, however, often difficult to apply in practice, practical optics is usually done using simplified models. The most common of these, geometric optics, treats light as a collection of rays that travel in straight lines, physical optics is a more comprehensive model of light, which includes wave effects such as diffraction and interference that cannot be accounted for in geometric optics. Historically, the model of light was developed first, followed by the wave model of light. Progress in electromagnetic theory in the 19th century led to the discovery that waves were in fact electromagnetic radiation. Some phenomena depend on the fact that light has both wave-like and particle-like properties, explanation of these effects requires quantum mechanics. When considering lights particle-like properties, the light is modelled as a collection of particles called photons, quantum optics deals with the application of quantum mechanics to optical systems. Optical science is relevant to and studied in many related disciplines including astronomy, various engineering fields, photography, practical applications of optics are found in a variety of technologies and everyday objects, including mirrors, lenses, telescopes, microscopes, lasers, and fibre optics. Optics began with the development of lenses by the ancient Egyptians and Mesopotamians, the earliest known lenses, made from polished crystal, often quartz, date from as early as 700 BC for Assyrian lenses such as the Layard/Nimrud lens. The ancient Romans and Greeks filled glass spheres with water to make lenses, the word optics comes from the ancient Greek word ὀπτική, meaning appearance, look. Greek philosophy on optics broke down into two opposing theories on how vision worked, the theory and the emission theory. The intro-mission approach saw vision as coming from objects casting off copies of themselves that were captured by the eye, plato first articulated the emission theory, the idea that visual perception is accomplished by rays emitted by the eyes. He also commented on the parity reversal of mirrors in Timaeus, some hundred years later, Euclid wrote a treatise entitled Optics where he linked vision to geometry, creating geometrical optics. Ptolemy, in his treatise Optics, held a theory of vision, the rays from the eye formed a cone, the vertex being within the eye. The rays were sensitive, and conveyed back to the observer’s intellect about the distance. He summarised much of Euclid and went on to describe a way to measure the angle of refraction, during the Middle Ages, Greek ideas about optics were resurrected and extended by writers in the Muslim world

Optics
–
Optics includes study of

dispersion of light.

Optics
–
The Nimrud lens

Optics
–
Reproduction of a page of

Ibn Sahl 's manuscript showing his knowledge of the law of refraction, now known as

Snell's law
Optics
–
Cover of the first edition of Newton's Opticks

2.
Laser science
–
Laser science or laser physics is a branch of optics that describes the theory and practice of lasers. Laser science predates the invention of the laser itself, the existence of stimulated emission was confirmed in 1928 by Rudolf W. Ladenburg. In 1939, Valentin A. Fabrikant predicted the use of stimulated emission to amplify short waves, In 1947, Willis E. Lamb and R. C. The theoretical principles describing the operation of a microwave laser were first described by Nikolay Basov, the first maser was built by Charles H. Townes, James P. Gordon, and H. J. Zeiger in 1953. Townes, Basov and Prokhorov were awarded the Nobel Prize in Physics in 1964 for their research in the field of stimulated emission, the first working laser was demonstrated on May 16,1960, by Theodore Maiman at the Hughes Research Laboratories. Laser acronyms List of laser types A very detailed tutorial on lasers

Laser science
–

United States Air Force laser experiment

3.
Light beam
–
A light beam or beam of light is a directional projection of light energy radiating from a light source. Sunlight forms a light beam when filtered through media such as clouds, foliage, to artificially produce a light beam, a lamp and a parabolic reflector is used in many lighting devices such as spotlights, car headlights, PAR Cans and LED housings. Light from certain types of laser has the smallest possible beam divergence. From the side, a beam of light is visible if part of the light is scattered by objects, tiny particles like dust, water droplets, hail, snow, or smoke. If there are objects in the light path, then it appears as a continuous beam. In any case, this scattering of light from a beam, flashlight, beam directed by hand Headlight, forward beam, the lamp is mounted in a vehicle, or on the forehead of a person, e. g. The difference between the two is that the fog itself is also a visual effect, Laser lighting display- Laser beams are often used for visual effects, often in combination with music. This also used to be done for movie premieres, the waving searchlight beams are still to be seen as an element in the logo of the 20th Century Fox movie studio

Light beam
–

A Symphony of Lights in

Victoria Harbour,

Hong Kong
Light beam
–
Light beams were used to symbolize the missing towers of the

World Trade Center as part of the

Tribute in Light.

Light beam
–
A natural lightbeam in the

Majlis al-Jinn (literally 'Meeting place of the

jinn ') cave in

Oman
Light beam
–
Beams shining through water-based haze in a photo studio setting.

4.
Beam waist
–
This fundamental transverse gaussian mode describes the intended output of most lasers, as such a beam can be focused into the most concentrated spot. When such a beam is refocused by a lens, the transverse dependence is altered. The electric and magnetic field amplitude profiles along any such circular Gaussian beam are determined by a single parameter, at any position z relative to the waist along a beam having a specified w0, the field amplitudes and phases are thereby determined as detailed below. The equations below assume a beam with a circular cross-section at all values of z, arbitrary solutions of the paraxial Helmholtz equation can be expressed as combinations of Hermite–Gaussian modes or similarly as combinations of Laguerre–Gaussian modes. At any point along the beam z these modes include the same Gaussian factor as the fundamental Gaussian mode multiplying the additional geometrical factors for the specified mode, Gaussian beam normally implies radiation confined to the fundamental Gaussian mode. The Gaussian beam is an electromagnetic mode. The mathematical expression for the field amplitude is a solution to the paraxial Helmholtz equation. There is also an understood time dependence e i ω t multiplying such phasor quantities, since this solution relies on the paraxial approximation, it is not accurate for very strongly diverging beams. In most practical cases the form is valid. Then the waves associated magnetic field is directly proportional to the electric field. For free space, η = η0 ≈377 Ω, at a position z along the beam, the spot size parameter w is given by w = w 01 +2. Where z R = π w 02 λ is called the Rayleigh range as further discussed below. The radius of the w, at any position z along the beam, is related to the full width at half maximum at that position according to. The curvature of the wavefronts is zero at the beam waist and it is equal to 1/R where R is the radius of curvature as a function of position along the beam, given by R = z. The so-called Gouy phase of the beam at z is given by, the Gouy phase results in an increase in the apparent wavelength near the waist. The phase velocity near the waist exceeds the speed of light in the medium, the Gouy phase shift along the beam remains within the range ±π/2 and is not observable in most experiments. However it is of importance and takes on a greater range for higher-order Gaussian modes. Many laser beams have an elliptical cross-section, also common are beams with waist positions which are different for the two transverse dimensions, called astigmatic beams

Beam waist
–
A 5 mW green laser pointer beam profile, showing the TEM 00 profile

Beam waist
–
Intensity of a Gaussian beam around focus at an instant of time, showing two intensity peaks for each

wavefront.

5.
Cross section (geometry)
–
In geometry and science, a cross section is the intersection of a body in three-dimensional space with a plane, or the analog in higher-dimensional space. Cutting an object into slices creates many parallel cross sections, conic sections – circles, ellipses, parabolas, and hyperbolas – are formed by cross-sections of a cone at various different angles, as seen in the diagram at left. Any planar cross-section passing through the center of an ellipsoid forms an ellipse on its surface, a cross-section of a cylinder is a circle if the cross-section is parallel to the cylinders base, or an ellipse with non-zero eccentricity if it is neither parallel nor perpendicular to the base. If the cross-section is perpendicular to the base it consists of two line segments unless it is just tangent to the cylinder, in which case it is a single line segment. A cross section of a polyhedron is a polygon, if instead the cross section is taken for a fixed value of the density, the result is an iso-density contour. For the normal distribution, these contours are ellipses, a cross section can be used to visualize the partial derivative of a function with respect to one of its arguments, as shown at left. In economics, a function f specifies the output that can be produced by various quantities x and y of inputs, typically labor. The production function of a firm or a society can be plotted in three-dimensional space, also in economics, a cardinal or ordinal utility function u gives the degree of satisfaction of a consumer obtained by consuming quantities w and v of two goods. Cross sections are used in anatomy to illustrate the inner structure of an organ. A cross section of a trunk, as shown at left, reveals growth rings that can be used to find the age of the tree. Cavalieris principle states that solids with corresponding sections of equal areas have equal volumes. The cross-sectional area of an object when viewed from an angle is the total area of the orthographic projection of the object from that angle. For example, a cylinder of height h and radius r has A ′ = π r 2 when viewed along its central axis, a sphere of radius r has A ′ = π r 2 when viewed from any angle. For a convex body, each ray through the object from the viewers perspective crosses just two surfaces, descriptive geometry Exploded view drawing Graphical projection Plans

Cross section (geometry)
–

Pinus taeda cross section showing annual rings,

Cheraw, South Carolina.

6.
Gaussian beam
–
This fundamental transverse gaussian mode describes the intended output of most lasers, as such a beam can be focused into the most concentrated spot. When such a beam is refocused by a lens, the transverse dependence is altered. The electric and magnetic field amplitude profiles along any such circular Gaussian beam are determined by a single parameter, at any position z relative to the waist along a beam having a specified w0, the field amplitudes and phases are thereby determined as detailed below. The equations below assume a beam with a circular cross-section at all values of z, arbitrary solutions of the paraxial Helmholtz equation can be expressed as combinations of Hermite–Gaussian modes or similarly as combinations of Laguerre–Gaussian modes. At any point along the beam z these modes include the same Gaussian factor as the fundamental Gaussian mode multiplying the additional geometrical factors for the specified mode, Gaussian beam normally implies radiation confined to the fundamental Gaussian mode. The Gaussian beam is an electromagnetic mode. The mathematical expression for the field amplitude is a solution to the paraxial Helmholtz equation. There is also an understood time dependence e i ω t multiplying such phasor quantities, since this solution relies on the paraxial approximation, it is not accurate for very strongly diverging beams. In most practical cases the form is valid. Then the waves associated magnetic field is directly proportional to the electric field. For free space, η = η0 ≈377 Ω, at a position z along the beam, the spot size parameter w is given by w = w 01 +2. Where z R = π w 02 λ is called the Rayleigh range as further discussed below. The radius of the w, at any position z along the beam, is related to the full width at half maximum at that position according to. The curvature of the wavefronts is zero at the beam waist and it is equal to 1/R where R is the radius of curvature as a function of position along the beam, given by R = z. The so-called Gouy phase of the beam at z is given by, the Gouy phase results in an increase in the apparent wavelength near the waist. The phase velocity near the waist exceeds the speed of light in the medium, the Gouy phase shift along the beam remains within the range ±π/2 and is not observable in most experiments. However it is of importance and takes on a greater range for higher-order Gaussian modes. Many laser beams have an elliptical cross-section, also common are beams with waist positions which are different for the two transverse dimensions, called astigmatic beams

Gaussian beam
–
A 5 mW green laser pointer beam profile, showing the TEM 00 profile

Gaussian beam

7.
Wavelength
–
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the waves shape repeats, and thus the inverse of the spatial frequency. Wavelength is commonly designated by the Greek letter lambda, the concept can also be applied to periodic waves of non-sinusoidal shape. The term wavelength is also applied to modulated waves. Wavelength depends on the medium that a wave travels through, examples of wave-like phenomena are sound waves, light, water waves and periodic electrical signals in a conductor. A sound wave is a variation in air pressure, while in light and other electromagnetic radiation the strength of the electric, water waves are variations in the height of a body of water. In a crystal lattice vibration, atomic positions vary, wavelength is a measure of the distance between repetitions of a shape feature such as peaks, valleys, or zero-crossings, not a measure of how far any given particle moves. For example, in waves over deep water a particle near the waters surface moves in a circle of the same diameter as the wave height. The range of wavelengths or frequencies for wave phenomena is called a spectrum, the name originated with the visible light spectrum but now can be applied to the entire electromagnetic spectrum as well as to a sound spectrum or vibration spectrum. In linear media, any pattern can be described in terms of the independent propagation of sinusoidal components. The wavelength λ of a sinusoidal waveform traveling at constant speed v is given by λ = v f, in a dispersive medium, the phase speed itself depends upon the frequency of the wave, making the relationship between wavelength and frequency nonlinear. In the case of electromagnetic radiation—such as light—in free space, the speed is the speed of light. Thus the wavelength of a 100 MHz electromagnetic wave is about, the wavelength of visible light ranges from deep red, roughly 700 nm, to violet, roughly 400 nm. For sound waves in air, the speed of sound is 343 m/s, the wavelengths of sound frequencies audible to the human ear are thus between approximately 17 m and 17 mm, respectively. Note that the wavelengths in audible sound are much longer than those in visible light, a standing wave is an undulatory motion that stays in one place. A sinusoidal standing wave includes stationary points of no motion, called nodes, the upper figure shows three standing waves in a box. The walls of the box are considered to require the wave to have nodes at the walls of the box determining which wavelengths are allowed, the stationary wave can be viewed as the sum of two traveling sinusoidal waves of oppositely directed velocities. Consequently, wavelength, period, and wave velocity are related just as for a traveling wave, for example, the speed of light can be determined from observation of standing waves in a metal box containing an ideal vacuum. In that case, the k, the magnitude of k, is still in the same relationship with wavelength as shown above

Wavelength
–
Wavelength is decreased in a medium with slower propagation.

Wavelength
–
Wavelength of a

sine wave, λ, can be measured between any two points with the same

phase, such as between crests, or troughs, or corresponding

zero crossings as shown.

Wavelength
–
Various local wavelengths on a crest-to-crest basis in an ocean wave approaching shore

Wavelength
–
A wave on a line of atoms can be interpreted according to a variety of wavelengths.

8.
Radian
–
The radian is the standard unit of angular measure, used in many areas of mathematics. The length of an arc of a circle is numerically equal to the measurement in radians of the angle that it subtends. The unit was formerly an SI supplementary unit, but this category was abolished in 1995, separately, the SI unit of solid angle measurement is the steradian. The radian is represented by the symbol rad, so for example, a value of 1.2 radians could be written as 1.2 rad,1.2 r,1. 2rad, or 1. 2c. Radian describes the angle subtended by a circular arc as the length of the arc divided by the radius of the arc. One radian is the angle subtended at the center of a circle by an arc that is equal in length to the radius of the circle. Conversely, the length of the arc is equal to the radius multiplied by the magnitude of the angle in radians. As the ratio of two lengths, the radian is a number that needs no unit symbol, and in mathematical writing the symbol rad is almost always omitted. When quantifying an angle in the absence of any symbol, radians are assumed, and it follows that the magnitude in radians of one complete revolution is the length of the entire circumference divided by the radius, or 2πr / r, or 2π. Thus 2π radians is equal to 360 degrees, meaning that one radian is equal to 180/π degrees, the concept of radian measure, as opposed to the degree of an angle, is normally credited to Roger Cotes in 1714. He described the radian in everything but name, and he recognized its naturalness as a unit of angular measure, the idea of measuring angles by the length of the arc was already in use by other mathematicians. For example, al-Kashi used so-called diameter parts as units where one part was 1/60 radian. The term radian first appeared in print on 5 June 1873, in examination questions set by James Thomson at Queens College, Belfast. He had used the term as early as 1871, while in 1869, Thomas Muir, then of the University of St Andrews, in 1874, after a consultation with James Thomson, Muir adopted radian. As stated, one radian is equal to 180/π degrees, thus, to convert from radians to degrees, multiply by 180/π. The length of circumference of a circle is given by 2 π r, so, to convert from radians to gradians multiply by 200 / π, and to convert from gradians to radians multiply by π /200. This is because radians have a mathematical naturalness that leads to a more elegant formulation of a number of important results, most notably, results in analysis involving trigonometric functions are simple and elegant when the functions arguments are expressed in radians. Because of these and other properties, the trigonometric functions appear in solutions to problems that are not obviously related to the functions geometrical meanings

Radian
–
A chart to convert between degrees and radians

Radian
–
An arc of a

circle with the same length as the

radius of that circle corresponds to an angle of 1 radian. A full circle corresponds to an angle of 2

π radians.

9.
Paraxial approximation
–
In geometric optics, the paraxial approximation is a small-angle approximation used in Gaussian optics and ray tracing of light through an optical system. A paraxial ray is a ray which makes an angle to the optical axis of the system. Generally, this allows three important approximations for calculation of the path, namely, sin θ ≈ θ, tan θ ≈ θ. The paraxial approximation is used in Gaussian optics and first-order ray tracing, ray transfer matrix analysis is one method that uses the approximation. In some cases, the approximation is also called paraxial. The approximations above for sine and tangent do not change for the second-order paraxial approximation, the second-order approximation is accurate within 0. 5% for angles under about 10°, but its inaccuracy grows significantly for larger angles. For larger angles it is necessary to distinguish between meridional rays, which lie in a plane containing the optical axis, and sagittal rays. Paraxial Approximation and the Mirror by David Schurig, The Wolfram Demonstrations Project

Paraxial approximation
–
The error associated with the paraxial approximation. In this plot the cosine is approximated by 1 - θ 2 /2.

10.
Physical optics
–
This usage tends not to include effects such as quantum noise in optical communication, which is studied in the sub-branch of coherence theory. Physical optics is also the name of a commonly used in optics, electrical engineering. In this context, it is an intermediate method between geometric optics, which ignores wave effects, and full wave electromagnetism, which is a precise theory. The word physical means that it is more physical than geometric or ray optics and this approximation consists of using ray optics to estimate the field on a surface and then integrating that field over the surface to calculate the transmitted or scattered field. This resembles the Born approximation, in that the details of the problem are treated as a perturbation, in optics, it is a standard way of estimating diffraction effects. In radio, this approximation is used to some effects that resemble optical effects. It models several interference, diffraction and polarization effects but not the dependence of diffraction on polarization, since it is a high-frequency approximation, it is often more accurate in optics than for radio. In optics, it consists of integrating ray-estimated field over a lens. Current on the parts is taken as zero. The approximate scattered field is obtained by an integral over these approximate currents. This is useful for bodies with large smooth convex shapes and for lossy surfaces, the ray-optics field or current is generally not accurate near edges or shadow boundaries, unless supplemented by diffraction and creeping wave calculations. The standard theory of optics has some defects in the evaluation of scattered fields. An improved theory introduced in 2004 gives exact solutions to problems involving wave diffraction by conducting scatterers, electromagnetic modeling History of optics Negative-index metamaterials Serway, Raymond A. Jewett, John W. Physics for Scientists and Engineers. A double-edge-diffraction Gaussian-series method for efficient physical optics analysis of dual-shaped-reflector antennas, IEEE Transactions on Antennas and Propagation,2597. The physical optics method in electromagnetic scattering

Physical optics
–
Physical optics is used to explain effects such as

diffraction.

11.
Gaussian function
–
In mathematics, a Gaussian function, often simply referred to as a Gaussian, is a function of the form, f = a e −22 c 2 for arbitrary real constants a, b and c. It is named after the mathematician Carl Friedrich Gauss, the graph of a Gaussian is a characteristic symmetric bell curve shape. The parameter a is the height of the peak, b is the position of the center of the peak. Gaussian functions arise by composing the exponential function with a quadratic function. The Gaussian functions are thus those functions whose logarithm is a quadratic function. The parameter c is related to the width at half maximum of the peak according to F W H M =22 ln 2 c ≈2.35482 c. The full width at tenth of maximum for a Gaussian could be of interest and is F W T M =22 ln 10 c ≈4.29193 c, Gaussian functions are analytic, and their limit as x → ∞ is 0. Gaussian functions are among those functions that are elementary but lack elementary antiderivatives and these Gaussians are plotted in the accompanying figure. Gaussian functions centered at zero minimize the Fourier uncertainty principle. The product of two Gaussian functions is a Gaussian, and the convolution of two Gaussian functions is also a Gaussian, with variance being the sum of the original variances, the product of two Gaussian probability density functions, though, is not in general a Gaussian PDF. Taking the Fourier transform of a Gaussian function with parameters a =1, b =0 and c yields another Gaussian function, so in particular the Gaussian functions with b =0 and c =1 are kept fixed by the Fourier transform. A physical realization is that of the pattern, for example. The integral ∫ − ∞ ∞ a e −2 /2 c 2 d x for some real constants a, b, c >0 can be calculated by putting it into the form of a Gaussian integral. First, the constant a can simply be factored out of the integral, next, the variable of integration is changed from x to y = x - b. Consequently, the sets of the Gaussian will always be ellipses. A particular example of a two-dimensional Gaussian function is f = A exp , here the coefficient A is the amplitude, xo, yo is the center and σx, σy are the x and y spreads of the blob. The figure on the right was created using A =1, xo =0, yo =0, σx = σy =1. The volume under the Gaussian function is given by V = ∫ − ∞ ∞ ∫ − ∞ ∞ f d x d y =2 π A σ x σ y

Gaussian function
–
Normalized Gaussian curves with

expected value μ and

variance σ 2. The corresponding parameters are, b = μ, and c = σ.

12.
Electromagnetic wave equation
–
The electromagnetic wave equation is a second-order partial differential equation that describes the propagation of electromagnetic waves through a medium or in a vacuum. It is a form of the wave equation. In a vacuum, vph = c0 =299,792,458 meters per second, the electromagnetic wave equation derives from Maxwells equations. It should also be noted that in most older literature, B is called the flux density or magnetic induction. To obtain the electromagnetic wave equation in a vacuum using the modern method, we begin with the modern Heaviside form of Maxwells equations. And ∇2 V = ∇ ⋅ where ∇V is a dyadic which when operated on by the divergence operator ∇ ⋅ yields a vector.99792458 ×108 m/s is the speed of light in free space. The electromagnetic wave equation is modified in two ways, the derivative is replaced with the covariant derivative and a new term that depends on the curvature appears. − A α, β, β + R α β A β =0 where R α β is the Ricci curvature tensor, the generalization of the Lorenz gauge condition in curved spacetime is assumed, A μ, μ =0. Localized time-varying charge and current densities can act as sources of waves in a vacuum. Maxwells equations can be written in the form of an equation with sources. The addition of sources to the wave equations makes the differential equations inhomogeneous. Although the function g can be and often is a sine wave, it does not have to be sinusoidal. In practice, g cannot have infinite periodicity because any real electromagnetic wave must always have an extent in time. As a result, and based on the theory of Fourier decomposition, the variable c can only be used in this equation when the electromagnetic wave is in a vacuum. Consider a plane defined by a normal vector n = k k. Then planar traveling wave solutions of the equations are E = E0 e − i k ⋅ r B = B0 e − i k ⋅ r where r = is the position vector. These solutions represent planar waves traveling in the direction of the vector n. Because the divergence of the electric and magnetic fields are zero and this solution is the linearly polarized solution of the wave equations

Electromagnetic wave equation
–
A postcard from Maxwell to

Peter Tait.

13.
John Strutt, 3rd Baron Rayleigh
–
John William Strutt, 3rd Baron Rayleigh OM PC PRS was a physicist who, with William Ramsay, discovered argon, an achievement for which he earned the Nobel Prize for Physics in 1904. He also discovered the now called Rayleigh scattering, which can be used to explain why the sky is blue. Rayleighs textbook, The Theory of Sound, is referred to by acoustic engineers today. John William Strutt, of Terling Place Essex, suffered from frailty and he attended Harrow School, before going on to the University of Cambridge in 1861 where he studied mathematics at Trinity College, Cambridge. He obtained a Bachelor of Arts degree in 1865, and a Master of Arts in 1868 and he was subsequently elected to a Fellowship of Trinity. He held the post until his marriage to Evelyn Balfour, daughter of James Maitland Balfour and he had three sons with her. In 1873, on the death of his father, John Strutt, 2nd Baron Rayleigh and he was the second Cavendish Professor of Physics at the University of Cambridge, from 1879 to 1884. He first described dynamic soaring by seabirds in 1883, in the British journal Nature, from 1887 to 1905 he was Professor of Natural Philosophy at the Royal Institution. Around the year 1900 Lord Rayleigh developed the theory of human sound localisation using two binaural cues, interaural phase difference and interaural level difference. The theory posits that we use two primary cues for sound lateralisation, using the difference in the phases of sinusoidal components of the sound, in 1919, Rayleigh served as President of the Society for Psychical Research. The rayl unit of acoustic impedance is named after him, as an advocate that simplicity and theory be part of the scientific method, Lord Rayleigh argued for the principle of similitude. Lord Rayleigh was elected Fellow of the Royal Society on 12 June 1873, from time to time Lord Rayleigh participated in the House of Lords, however, he spoke up only if politics attempted to become involved in science. He died on 30 June 1919, in Witham, Essex and he was succeeded, as the 4th Lord Rayleigh, by his son Robert John Strutt, another well-known physicist. Lord Rayleigh was buried in the graveyard of All Saints Church in Terling in Essex, though he did not write about the relationship of science and religion, he retained a personal interest in spiritual matters. The Secretary to the Press suggested with many apologies that the reader might suppose that I was the Lord, still, he kept his wish and the quotation was printed in the five-volume collection of scientific papers. What is more, I think that Christ and indeed other spiritually gifted men see further and truer than I do, Lord Rayleigh was the president of the SPR in 1919. He gave an address the year of his death but did not come to any definite conclusions. Craters on Mars and the Moon are named in his honour as well as a type of surface wave known as a Rayleigh wave, the asteroid 22740 Rayleigh was named in his honour on 1 June 2007

John Strutt, 3rd Baron Rayleigh
–
The Lord Rayleigh

14.
Robert Strutt, 4th Baron Rayleigh
–
Robert John Strutt, 4th Baron Rayleigh FRS was a British peer and physicist. He discovered active nitrogen and was the first to distinguish the glow of the night sky, Strutt was born at Terling Place, the family home near Witham, Essex, the eldest son of John William Strutt, 3rd Baron Rayleigh and his wife Evelyn Georgiana Mary. He was thus a nephew of Arthur Balfour and of Eleanor Mildred Sidgwick and he was educated at Eton College and Trinity College, Cambridge, where he initially read mathematics, but changed after two terms to Natural Sciences. He became a student in physics at the Cavendish Laboratory under J. J. Thomson. His work at this time was on discharge of electricity through gases, including work on x-rays. He wrote one of the first books on radioactivity, The Becquerel rays and he was awarded the Coutts Trotter studentship in 1898 and was a Fellow of Trinity College 1900–1906. He received his M. A. in 1901, Strutt was elected a Fellow of the Royal Society in May,1905 when his candidature citation read, Fellow of Trinity College, Cambridge. He delivered their Bakerian Lecture in 1911 and 1919 and he was president of the British Association for the year 1937–1938. Strutts best known work in the period 1904–1910 was the estimation of the age of minerals and he published a biography of his father, the third Baron Rayleigh, with the title of Life of John William Strutt, third Baron Rayleigh. Photographs of Robert John Strutt when he was young can be found in this book, both the father and sons work on light scattering was discussed by Young in 1982. A sketch of Robert John Strutt when he was old can also be found in Young, in 1910 Robert Strutt discovered that an electrical discharge in nitrogen gas produced active nitrogen, an allotrope considered to be monatomic. The whirling cloud of brilliant yellow light produced by his apparatus reacted with quicksilver to produce explosive mercury nitride. In 1916, working with his colleague Alfred Fowler, Strutt was the first to prove the existence of ozone in the atmosphere by examining the spectrum of the setting sun. Strutt proved that the ozone was mainly located in the upper atmosphere and his earlier work on gaseous discharge and fluorescence, led to further work on the luminosity of the night sky. He was the first to differentiate two types of light from the night sky, the aurora or northern lights, and the airglow that prevents the sky ever being completely dark anywhere on earth. In 1929 he was the first to measure the intensity of the light from the night sky and this work led to his posthumous nickname the Airglow Rayleigh. They are now housed in the McDermott Library of the U. S. Air Force Academy, Colorado Springs, the rayleigh, a unit of photon flux used to measure airglow, is named after him. A special issue of Applied Optics published in 1964 is devoted to the 3rd, Strutt inherited his title on the death of his father in 1919, becoming the 4th Baron Rayleigh

Robert Strutt, 4th Baron Rayleigh
–
Strutt with his son in 1938

Robert Strutt, 4th Baron Rayleigh
–
Robert Strutt

15.
Depth of field
–
In some cases, it may be desirable to have the entire image sharp, and a large DOF is appropriate. In other cases, a small DOF may be more effective, in cinematography, a large DOF is often called deep focus, and a small DOF is often called shallow focus. Precise focus is possible at one distance, at that distance. At any other distance, a point object is defocused, and will produce a blur spot shaped like the aperture, when this circular spot is sufficiently small, it is indistinguishable from a point, and appears to be in focus, it is rendered as acceptably sharp. The acceptable circle of confusion is influenced by visual acuity, viewing conditions, the increase of the circle diameter with defocus is gradual, so the limits of depth of field are not hard boundaries between sharp and unsharp. For 35 mm motion pictures, the area on the film is roughly 22 mm by 16 mm. The limit of tolerable error was traditionally set at 0.05 mm diameter, while for 16 mm film, where the size is half as large. More modern practice for 35 mm productions set the circle of confusion limit at 0.025 mm, for full-frame 35mm still photography, the circle of confusion is usually chosen to be about 1/30 mm. Many sources propose CoC limits as a fraction of the film format diagonal, the three formats above at fraction 1/1500 would use 0.029,0.056, and 0.017 mm. Traditional depth-of-field formulas and tables assume equal circles of confusion for near and far objects, the loss of detail in distant objects may be particularly noticeable with extreme enlargements. Achieving this additional sharpness in distant objects usually requires focusing beyond the hyperfocal distance, with this approach, foreground objects cannot always be made perfectly sharp, but the loss of sharpness in near objects may be acceptable if recognizability of distant objects is paramount. Other authors have taken the position, maintaining that slight unsharpness in foreground objects is usually more disturbing than slight unsharpness in distant parts of a scene. The combination of length, subject distance, and format size defines magnification at the film / sensor plane. DOF is determined by subject magnification at the film / sensor plane, for a given f-number, increasing the magnification, either by moving closer to the subject or using a lens of greater focal length, decreases the DOF, decreasing magnification increases DOF. For a given magnification, increasing the f-number increases the DOF. If the original image is enlarged to make the final image, when focus is set to the hyperfocal distance, the DOF extends from half the hyperfocal distance to infinity, and the DOF is the largest possible for a given f-number. The comparative DOFs of two different format sizes depend on the conditions of the comparison, the DOF for the smaller format can be either more than or less than that for the larger format. In the discussion that follows, it is assumed that the images from both formats are the same size, are viewed from the same distance, and are judged with the same circle of confusion criterion

Depth of field
–
The area within the depth of field appears sharp, while the areas beyond the depth of field appear blurry.

Depth of field
–
A

macro photograph with very shallow depth of field

Depth of field
–
A

macro photograph of a

Nokia 101 phone with extremely shallow depth of field of only a few millimeters and a strong

bokeh effect.

Depth of field
–
A 35 mm lens set to

*f*/11. The depth-of-field scale (top) indicates that a subject which is anywhere between 1 and 2 meters in front of the camera will be rendered acceptably sharp. If the aperture were set to f /22 instead, everything from just over 0.7 meters almost to infinity would appear to be in focus.

16.
International Standard Book Number
–
The International Standard Book Number is a unique numeric commercial book identifier. An ISBN is assigned to each edition and variation of a book, for example, an e-book, a paperback and a hardcover edition of the same book would each have a different ISBN. The ISBN is 13 digits long if assigned on or after 1 January 2007, the method of assigning an ISBN is nation-based and varies from country to country, often depending on how large the publishing industry is within a country. The initial ISBN configuration of recognition was generated in 1967 based upon the 9-digit Standard Book Numbering created in 1966, the 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO2108. Occasionally, a book may appear without a printed ISBN if it is printed privately or the author does not follow the usual ISBN procedure, however, this can be rectified later. Another identifier, the International Standard Serial Number, identifies periodical publications such as magazines, the ISBN configuration of recognition was generated in 1967 in the United Kingdom by David Whitaker and in 1968 in the US by Emery Koltay. The 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO2108, the United Kingdom continued to use the 9-digit SBN code until 1974. The ISO on-line facility only refers back to 1978, an SBN may be converted to an ISBN by prefixing the digit 0. For example, the edition of Mr. J. G. Reeder Returns, published by Hodder in 1965, has SBN340013818 -340 indicating the publisher,01381 their serial number. This can be converted to ISBN 0-340-01381-8, the check digit does not need to be re-calculated, since 1 January 2007, ISBNs have contained 13 digits, a format that is compatible with Bookland European Article Number EAN-13s. An ISBN is assigned to each edition and variation of a book, for example, an ebook, a paperback, and a hardcover edition of the same book would each have a different ISBN. The ISBN is 13 digits long if assigned on or after 1 January 2007, a 13-digit ISBN can be separated into its parts, and when this is done it is customary to separate the parts with hyphens or spaces. Separating the parts of a 10-digit ISBN is also done with either hyphens or spaces, figuring out how to correctly separate a given ISBN number is complicated, because most of the parts do not use a fixed number of digits. ISBN issuance is country-specific, in that ISBNs are issued by the ISBN registration agency that is responsible for country or territory regardless of the publication language. Some ISBN registration agencies are based in national libraries or within ministries of culture, in other cases, the ISBN registration service is provided by organisations such as bibliographic data providers that are not government funded. In Canada, ISBNs are issued at no cost with the purpose of encouraging Canadian culture. In the United Kingdom, United States, and some countries, where the service is provided by non-government-funded organisations. Australia, ISBNs are issued by the library services agency Thorpe-Bowker

International Standard Book Number
–
A 13-digit ISBN, 978-3-16-148410-0, as represented by an

EAN-13 bar code